1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, it relates to a semiconductor device including an SiO2 film (silicon oxide film) applied to a gate insulator film or a tunnel insulator film.
2. Description of the Background Art
A liquid crystal display or the like has recently been watched with interest as a lightweight display allowing miniaturization. This display, requiring a high voltage of about 15 V for driving, generally employs a control LSI (large scale integrated circuit) consisting of a reliable field-effect transistor (voltage-resistant transistor) unbreakable with a load of a high voltage and a fine transistor (low-voltage transistor) for implementing low power consumption and a high-speed operation.
In order to improve electric strength of the voltage-resistant transistor for attaining high reliability, it is generally effective to improve insulation properties of a gate insulator film held between a gate electrode constituting the voltage-resistant transistor and a silicon substrate. The insulation properties of the gate insulator film are generally improved by increasing the thickness of the gate insulator film consisting of SiO2.
An STI (shallow trench isolation) technique of forming a shallow trench on the surface of a substrate by dry etching and thereafter embedding an insulator in the trench thereby isolating elements from each other has recently been widely employed. When STI is employed for element isolation of a voltage-resistant transistor and a gate insulator film of the voltage-resistant transistor consisting of a thick SiO2 film is formed by thermal oxidation, however, the thickness of the gate insulator film is reduced on an upper corner of a trench formed by STI, disadvantageously leading to reduction of voltage resistance of the transistor. Therefore, it is effective to form the gate insulator film of the voltage-resistant transistor by CVD (chemical vapor deposition) capable of covering the overall surface including the upper corner of the trench formed by STI with a uniform thickness.
However, a gate insulator film formed by CVD or a thin gate insulator film formed by thermal oxidation is generally inferior in quality. In an initial stage of formation of the gate insulator film, an incomplete SiO2 film is easily formed due to a natural oxide film or a structural transition layer. This incomplete SiO2 film contains dangling bonds not completely in the form of O—Si—O. In the gate insulator film formed by CVD or the thin gate insulator film formed by thermal oxidation, therefore, a large number of electron traps are disadvantageously formed when electrons are continuously injected into the gate insulator film. Consequently, the number of electrons captured by the electron traps is increased with the time for electron injection, and hence the threshold voltage of the voltage-resistant transistor is disadvantageously remarkably changed. Thus, the time for reaching the maximum value of allowable threshold voltage variations is reduced in view of the characteristics of the voltage-resistant transistor, to disadvantageously reduce operational reliability (working life).
On the other hand, a nonvolatile memory such as an EPROM (erasable and programmable read only memory) or an EEPROM (electrically erasable and programmable read only memory) has recently been watched with interest as a semiconductor memory capable of replacing a hard disk or a floppy disk which is a magnetic memory.
The EPROM or the EEPROM stores data in response to presence/absence of electrons stored in a floating gate electrode forming a memory cell. Further, the EPROM or the EEPROM reads data by detecting change of a threshold voltage responsive to presence/absence of electrons stored in the floating gate electrode. In relation to the EEPROM, known is a flash EEPROM entirely erasing data in a memory cell array or dividing a memory cell array into arbitrary blocks and collectively erasing data in units of the blocks. The flash EEPROM, referred to as a flash memory, has excellent characteristics such as a large capacity, low power consumption, a high speed and shock resistance. Therefore, the flash memory is applied to various portable instruments. Further, each memory cell is constituted by a single transistor, whereby the flash memory can be easily highly integrated.
In general, a stacked gate memory cell structure is known as the structure of memory cells forming a flash memory. In each memory cell of such a stacked gate flash memory, a source region and a drain region are provided on the surface of a semiconductor substrate to hold a channel region therebetween at a prescribed interval. A floating gate electrode is provided on the channel region through a tunnel insulator film. A control gate electrode is formed on the floating gate electrode through a gate insulator film.
In order to write data in the stacked gate flash memory, a voltage of 10-odd V is applied to the control gate electrode while applying a voltage to the drain region for converting electrons flowing in the channel region of the semiconductor substrate to hot electrons. Thus, the hot electrons are injected into the floating gate electrode, thereby writing data. In order to erase data, a voltage of 10-odd V is applied to the source region. Thus, a Fowler-Nordheim tunnel current (F-N tunnel current) is fed from the source region toward the floating gate electrode, thereby extracting electrons stored in the floating gate electrode through the tunnel insulator film. In order to read data, the flash memory detects a current (cell current), flowing between the source region and the drain region, varying with presence/absence of electrons stored in the floating gate electrode, thereby determining the data.
Thus, the memory cell of the conventional stacked gate flash memory injects hot electrons into the floating gate electrode in writing, and utilizes an F-N tunnel current for extracting the electrons stored in the floating gate electrode in erasing.
In general, a thin SiO2 film formed by thermal oxidation is employed as the tunnel insulator film of the flash memory. However, the thin SiO2 film disadvantageously contains a large quantity of incomplete SiO2, as hereinabove described. When the flash memory applies a voltage to the source region for erasing data, therefore, electrons accelerated in a high electric field pass through the tunnel insulator film containing incomplete SiO2, to apply remarkable stress to the tunnel insulator film. Consequently, a large number of electron traps are disadvantageously formed in the tunnel insulator film. The electron traps formed in the tunnel insulator film inhibit transfer of electrons from the floating gate electrode to the source region in data erasing, leading to insufficient extraction of the electrons from the floating gate electrode. Following increase of the numbers of data writing and erasing, the number of electron traps is also increased due to the incomplete SiO2, to result in further reduction of the quantity of electrons extracted from the floating gate electrode. Thus, the number of electrons stored in the floating gate electrode after erasing is so increased that the cell current is disadvantageously reduced in data reading in an erased state. In this case, difference between the values of the cell current in writing and erasing is reduced as the numbers of data writing and erasing are increased, leading to difficulty in determination of data. In general, therefore, it is difficult to increase the number of data rewriting, and it is consequently difficult to improve the working life of the flash memory.